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Sustainably sourced components to generate high-strength adhesives

Nature volume 621, pages 306–311 (2023)Cite this article

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Abstract

Nearly all adhesives1,2 are derived from petroleum, create permanent bonds3, frustrate materials separation for recycling4,5 and prevent degradation in landfills. When trying to shift from petroleum feedstocks to a sustainable materials ecosystem, available options suffer from low performance, high cost or lack of availability at the required scales. Here we present a sustainably sourced adhesive system, made from epoxidized soy oil, malic acid and tannic acid, with performance comparable to that of current industrial products. Joints can be cured under conditions ranging from use of a hair dryer for 5 min to an oven at 180 °C for 24 h. Adhesion between metal substrates up to around 18 MPa is achieved, and, in the best cases, performance exceeds that of a classic epoxy, the strongest modern adhesive. All components are biomass derived, low cost and already available in large quantities. Manufacturing at scale can be a simple matter of mixing and heating, suggesting that this new adhesive may contribute towards the sustainable bonding of materials.

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Fig. 1: Adhesive chemistry.
Fig. 2: Performance of a sustainably sourced adhesive.
Fig. 3: Characterization of soy-mal-tan.
Fig. 4: Potential practical implications when using a sustainably sourced adhesive.

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Data availability

Data generated during the current study are available from the corresponding author on request.

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Acknowledgements

We thank P. Zavattieri and F. B. Rodriguez from the Lyles School of Civil Engineering at Purdue University for use of their MTS Insight instrument for adhesion testing. Help with microscopy by M. Meger, R. Seiler and C. Gilpin at the Purdue Life Science Microscopy Facility is appreciated. H. Siebert contributed to the initial experiments for this project. This work was supported by Office of Naval Research grant nos. N00014-19-1-2342 and N00014-22-1-2408.

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Author notes

  1. These authors contributed equally: Clayton R. Westerman, Bradley C. McGill

Authors and Affiliations

  1. Department of Chemistry, Purdue University, West Lafayette, IN, USA

    Clayton R. Westerman, Bradley C. McGill & Jonathan J. Wilker

  2. School of Materials Engineering, Purdue University, Neil Armstrong Hall of Engineering, West Lafayette, IN, USA

    Jonathan J. Wilker

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Contributions

C.R.W. and B.C.M. performed experiments. J.J.W. oversaw the project. The paper was written by all of the authors.

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Correspondence to Jonathan J. Wilker.

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Extended data figures and tables

Extended Data Fig. 1 Structures of classic epoxy chemistry and candidate components for bio-based adhesives.

a, Representation of reactivity in a classic epoxy adhesive. Amine nucleophiles react with the three-membered epoxy rings to form covalent cross-links. The cross-linked product here is a depiction of a more extensive matrix. b, Nucleophiles and phenolics that were reacted with epoxidized soy oil to generate several adhesives formulations. In each case, there was one nucleophile, one phenolic, and epoxidized soy oil. The structures shown for lignin and tannic acid are approximate.

Extended Data Fig. 2 Setup for measuring lap shear adhesion.

Two wood substrates are bonded together with an adhesive and then placed into the materials testing system. Each substrate has pin holes at the ends. One pin holds the bottom substrate in place. The top pin is attached to the moving crosshead and goes through the upper substrate. As the crosshead moves up and force is applied, a load cell measures force. The recorded force at joint failure is then divided by the substrate overlap area (1.2 x 1.2 cm here) to generate adhesion values in MPa.

Extended Data Fig. 3 Adhesive strengths of various ratios, times, and conditions.

a, Adhesion as a function of varied ratios between the components epoxidized soy oil, glycerol, and tannic acid. The substrates were untreated aluminum and curing was at 180 °C for 24 h. b, Adhesion as a function of varied ratios between the components epoxidized soy oil, malic acid, and tannic acid. The substrates were untreated aluminum and curing was at 180 °C for 24 h. c, Adhesion of soy-mal-tan over time when cured at 180 °C and with polished steel substrates. d, Adhesion of soy-mal-tan with changes to substrates and cure conditions. All error bars in panels a–c are 90% confidence intervals averaged from n = 5 samples with the exception of n = 10 samples for the 24 hour time point in panel c. The ± values in panel d are 90% confidence intervals from an average of n = 5 samples for the 6 hour, 180 °C cure. All other data are from n = 10 samples.

Extended Data Fig. 4 Typical force-versus-extension curves when measuring performance of adhesives.

a, The commercial products Super Glue and an epoxy are shown. b, Curves for soy-mal-tan cured at room temperature for 24 h, 70 °C for 24 h, and 180 °C for 6 h. All substrates here were polished steel.

Extended Data Fig. 5 Scanning electron microscopy images of adhesives after being pulled to failure.

a, A commercial epoxy shows clean fracture and distinct regions of adhesive versus substrate. b, The soy-mal-tan material shows more complex failure, with stress lines, indicative of ductile behavior. Both substrates were polished steel. The soy-mal-tan adhesive was cured at 180 °C for 6 h.

Extended Data Fig. 6 Appearance of the soy-mal-tan system at different stages.

a, Epoxidized soy oil, malic acid, and tannic acid upon initial mixing at room temperature. b, Soy-mal-tan after 24 h reaction time at 70 °C. Here the adhesive precursor was maintained at 70 °C and viscous, but flowing. c, After the 24 h reaction at 70 °C, cooling to room temperature brought about an increase in viscosity. d, Hardening after a 24 h cure at 180 °C.

Extended Data Fig. 7 Progressive pictures of epoxide titration.

a, Initial solution with methyl violet indicator. b, Approximate half-equivalence point reached. c, Equivalence point reached when light green color was present.

Extended Data Fig. 8 IR spectra of soy-mal-tan and controls.

a, Infrared spectrum of the final adhesive with all components, epoxidized soy oil, malic acid, and tannic acid. b, Infrared spectrum after a reaction between epoxidized soy oil and malic acid. c, Infrared spectrum of malic acid. Boxes highlight the CO–OH peaks in panels b and c.

Extended Data Fig. 9 Differential scanning calorimetry thermal traces for soy-mal-tan and controls.

Each plot is on the same scale, but offset from each other for comparisons.

Extended Data Fig. 10 Water resistance of soy-mal-tan and a commercial epoxy.

a, Resistance of soy-mal-tan to artificial sea water. Bonded pairs of polished aluminum substrates, with 1.2 x 1.2 cm overlap area, were cured in air for 24 h at 70 °C or 6 h at 180 °C and then submerged underwater for varied periods of time at room temperature. The x axis is a log plot in minutes, labelled in hours for clarity. b, Resistance of a commercial epoxy to artificial sea water. Bonded pairs of polished aluminum substrates were cured in air according to the manufacturer’s instructions and then submerged underwater for varied periods of time at room temperature. The x axis is a log plot in minutes, labelled in hours for clarity. c, Testing resistance of soy-mal-tan adhesion to boiling water. These substrates were polished aluminum. In the plots error bars are 90% confidence intervals. For panel a the 180 °C data are from n = 5 samples and the 70 °C data are from n = 10 samples. In panel b the 0 and 1 hour time points are from n = 5 samples with n = 10 samples for the 24 and 168 hour time points.

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Westerman, C.R., McGill, B.C. & Wilker, J.J. Sustainably sourced components to generate high-strength adhesives. Nature 621, 306–311 (2023). https://doi.org/10.1038/s41586-023-06335-7

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